1. Field of the Invention
The present invention relates to an image forming device such as a copying machine, a printer, a facsimile machine, or a multi-functional peripheral machine which is a combination of two or more of these, especially to a developing device used for the image forming device, and further especially to a two component developing device which is capable of forming a toner image by developing an electrostatic latent image formed on an electrostatic latent image carrier with a developer containing toner and magnetic carrier particles in the image forming device.
2. Description of Related Art
In an image forming device such as a copying machine, a printer, a facsimile machine, or a multi-functional peripheral machine which is a combination of two or more of these, a common two-component developing device which is capable of forming a toner image by developing an electrostatic latent image formed on an electrostatic latent image carrier with a developer containing toner and magnetic carrier particles comprises a development housing for containing a developer, a developing roller which is provided on the development housing and can be rotationally driven, a magnetic field generator which is mounted inside the developing roller, and has a plurality of magnetic poles for attracting developer in a development housing used for development onto the surface of the developing roller and meanwhile for providing a low-magnetic force area where the developer returned into the development housing without being used in development in a state of being left attracted on the developing roller is separated from the developing roller, and a developer restricting member which restricts the amount of the developer attracted onto the surface of the developing roller by the magnetic force of the magnetic field generator and transferred to an electrostatic latent image development area by the rotation of the developing roller, and opposes the developing roller across a developer amount restricting clearance.
FIG. 5 shows an example of such a conventional developing device.
A two-component developing device 4′ shown in FIG. 5 comprises a development housing 40′ for containing a developer D, a developing roller 41′ which is provided on the development housing 40′ and can be rotationally driven, a magnetic field generator 42′ mounted inside the developing roller 41′, an a developer restricting member 43′ opposing the developing roller 41′.
The developing device 4′ further comprises a rotationally driven developer transfer member 401′ which transfers the developer D in the development housing 40′ and at the same time distributes the same entirely to the developing roller 41′.
As shown in FIG. 5, the developer transfer member 401′ is generally disposed with its rotational axis aligned to the developing roller 41′.
A developing device provided with another developer transfer member (not shown) disposed therein which reciprocatingly circulates and transfers the developer by this another transfer member and the transfer member 401′ and distributes the developer to the developing roller 41′ is also widely known.
The magnetic field generator 42′ has a plurality of magnetic poles which attract the developer used in development in the development housing 40′ onto the surface of the developing roller 41′, and meanwhile provide a low-magnetic force area LM′ where the developer returned into the development housing 40′ without being used in development in a state of being left attracted is separated from the developing roller 41′.
In the example of FIG. 5, the magnetic field generator 42′ has, although not so limited, a catch pole S2 which attracts the developer D onto the developing roller 41′, a restricting pole N2 located in a position corresponding to the restricting member 43′, a transfer pole S1 for transferring the developer which has passed the restricting member 43′ to a development area Da′ where an electrostatic latent image on the electrostatic latent image carrier 1′ is developed, and a developer separating pole S3 which forms a repulsion magnetic field between the development pole N1 corresponding to the development area Da′ and the catch pole S2 to form the low-magnetic force area LM′.
The developer attracted onto the surface of the developing roller 41′ by the magnetic force of the magnetic field generator 42′ is transferred to a developer amount restricting clearance where the developer restricting member 43′ opposes the developing roller 41′ under the action of the frictional force by the rotation of the developing roller 41′, and is restricted to a predetermined amount in the clearance and transferred to the electrostatic latent image development area Da′, and is subjected to the development of an electrostatic latent image on the electrostatic latent image carrier 1′.
The developer which was not consumed in the development area Da′ and returned to the development housing 40′ in a state of being retained on the developing roller 41′ peels off from the developing roller in the low-magnetic force area LM′. The low-magnetic force area LM′ has a magnetic flux density of, for example, 5 mT or lower on the surface of the developing roller 41′.
In either case, the normal reaction which generates the frictional force for transferring developer by the rotation of the developing roller is mainly a component of the magnetic force due to the magnetic field generated by the magnetic field generator which is mounted inside the developing roller in the radial direction of the developing roller. The distribution of magnetic flux density on the surface of the developing roller which has a commonly used magnetic field generator thereinside to generate this normal reaction required for transferring developer, in other words, a component of the magnetic force of the magnetic field generator in the radial direction of the developing roller is approximately a few tens to a hundred mT.
Regarding this point, JP2008-15197A describes the following developing device. That is, the document describes that, to improve the ability of the developing roller to transfer developer, a developer slip restricting member is disposed in a manner of opposing and being spaced away from the developing roller in an upstream area of the developer restricting member. The developer is pressed against the developing roller by this component to suppress slipping of the developer with respect to the developing roller, in other words, the developer is pressed against the developing roller to ensure the frictional force required for transfer of the developer between the developing roller and the developer.
In a conventional two-component developing device as described above, except for the developing device described in JP2008-15197A, the amount of the developer attracted onto the developing roller by the magnetic force of the magnetic field generator is varied depending on change of the bulk of the developer and the position of a developer transfer member such as the above-mentioned developer transfer member 401′ which is usually provided on a two-component developing device and uneven transfer of the developer. This variation of the amount of the developer on the developing roller may lower the quality of images developed and formed.
Therefore, as an effective measure for equalizing the amount of the developer transferred to the development region without being affected by the variation of the amount of the developer attracted onto the developing roller, a measure is employed in many developing devices which supplies developer to the developing roller in an amount larger than that to be transferred to the development region (having the developing roller attracting such an amount of developer) and restricts the amount of the developer under a certain pressure by a developer restricting member.
However, the high pressure which acts on the developer in the restricting portion by the developer restricting member is a stress for the developer, which deteriorates the developer in the use of the developing device for a long period of time. For this reason, it is required to set the pressure acting on the developer in the restricting portion to a low level.
Described more specifically, developer, when its movement toward the development region is restricted by the developer restricting member, is bound close to the developing roller by magnetic force. Therefore, an accumulating portion of the developer is produced in an upstream area of the developer restricting member in the direction of rotation of the developing roller (in the device 4′ in FIG. 5, upstream area Ar), where the stress on the developer is increased.
By reducing the amount of the developer attracted onto the developing roller, the amount of the developer which needs to be restricted can be also reduced. However, the amount of the developer accumulating at the restricting member is determined by the structural conditions and magnetic force around the developer restricting portion. Therefore, as long as restriction of the amount is performed in the restricting portion, the amount is:
Amount of the developer on the developing roller before passing the restricting member>amount of the developer after passing the restricting member
Consequently, the amount of the developer accumulating cannot be reduced, and reducing stress of the developer is thus difficult.
The developing device 4″ shown in FIG. 6 is a variation of the developing device 4′ shown in FIG. 5 where the space Ar′ on the upstream side of the developer restricting member 43′ is set smaller. By setting the space on the upstream side of the developer restricting member 43′ smaller in such a manner, the accumulating portion of the developer is unlikely to be produced on the upstream side of the restricting member, and the amount of the developer which receives stress before being restricted is reduced. However, on the contrary, the developer is packed and the stress exerted on each (individual) particle constituting the developer is increased.
In a developing device of the type described in JP2008-15197A, the slip restricting member and the developer restricting member are spaced away from each other, whereby pressure is released so that the stress on the developer in the restricting portion can be suppressed. However, the amount of the developer fed is restricted at the upstream end of the slip restricting member (the upstream end in the direction of rotation of the developing roller), and the developer is therefore accumulated in the portion and the stress on the developer is generated. Such stress on the developer also occurs in this type of device.